Amplifier



Patented Mar. 9, 1943 UNITED STATES AMPLIFIER Wilson M. Brubalrcr,Wilkinsburg, Pa., assignor to Westinghouse Electric 8; ManufacturingGompm, East Pittsburgh,'la., a corporation or Pennsylvania Applicationmy 31, 1940, serial a... 332,111

lClaim. 01. 179171) This invention relates to amplifiers and, moreparticularly, to vacuum tube amplifiers suitable for the amplificationof voltage or current changes in direct as well as alternating currentcircuits.

The uselof vacuum tubesior the amplification of steady or slowly varyingpotentials has been very limited due to the dimculties in the design ofcircuits for this purpose. The primary difiiculty is found in thecoupling of amplifier stages between the output of one tube and theinput of 10 a succeeding tube. For this reason, amplifiers of signalswhich have slowly changing components have been limited largely tosingle vacuum tubes used in connection with a sensitive meter. Sinceoperating voltages of an amplifier include the application of positivepotentials between cathode and anode, whereas the potentials betweencathode and control electrode extend into a negative polarity range, theanode of one tube cannot'be connected directly in the manner conductiveto direct current to the control element of a succeeding tube if thecathodes of all the tubes are to be maintained at a uniform potential. Adirect connection in such circumstance would apply the relatively high.positive potential of the anode to the negatively biased control grid ofthe succeeding tube with detrimental results.

- In order to effect coupling between. amplifier stages, it is thepractice to insert a'condenser in the connection between anode and gridwhich isothe signalvoltage to be amplified. Furthermore,

an amplifier with capacitive coupling is limited-to alternating currentsignal voltages. Various circuits have been devised in the past foramplifiers to operate on direct current or slowly varying inputpotentials. In these the coupling capacity is usually replaced bybatteries or the stages have to be cascaded in such manner that thecathodes of the tubes are at difierent potentials with respect to eachother. While such arrangements circuits are extremely critical andrequire a number of isolated power supplies and very high voltages. Thelarge number of circuit components also-seriously affect the stabilityof the entire A particular feature of this invention is the directconductive connection between output and input elements of cascadedvacuum tubeamplifier stages with all the cathodes of the tubes at uni-'form potential. This permits slowly varying or 55' constant (directcurrent) changes of signal input voltages to be amplified, as well ahigher frequencies, extending well beyond'the audio frequency spectrum.

Another important feature of. this invention is the simple circuitarrangement utilizing a' minimum number ofcomponents fortheamplifierwhich is capable of direct current or alternating current signalamplification. I

A iurther advantage of the amplifier con-' stru'cted in accordance withthis invention is the stability of the circuit with only normalfiltering and regulation of the supply voltage which may be derived froma common power supply for all tubes and the adaptability to utilizeinverse feedback between stages.

Other features and advantages will be apparent from the followingdescription of the invention, pointed out in particularity in theappended 20 claim, and taken in connection with the accom- Fig. 3 showsa complete circuit of a power line,

perated amplifier of three cascaded stages, whereas l Fig. 4 is themodified circuit of the above with inclusion of inverse feedback.

Identical elements in the various figures of the drawings are identifiedwith similar reference characters, I r

Referring to the drawings, in Fig. 1 a simple amplifier circuit includesthe input terminals across potentiometer t, the lead 2 from whichconnects to the grid 3 of amplifying tube 5. The suitable biasingpotential for, the grid 3 is obtained by means of; potentiometer 5connected between the cathode ,Tof tube 5 and the supply conductor 9.The rider iii of the last-mentioned potentiometer connects to the groundpotential permit direct coupling of successive stages, the

ance M which terminates atthe anode potential supply conductor IE5.Between the anod l2 and system.

fier tube is efiected at the Junction point of resistors l6 and I! towhich the grid 20 is directly 19 connects to the common supply conductor22 which interconnects all cathodes and terminates at the commonconnection between the anode power supply B" and the bias power supplyC. The current flow which is desirable through the series connectedresistors l6, l'l-and l8 may be controlled by the variable contact 23 ofthe resistor l8 which is connected to the most nega-- tive portion ofthesystem represented by the supply conductor 9. The anode 25 of thetube l9 connects through a suitable load impedance device, shown here bythe relay winding 28120 the highest positive potential side of thesystem represented by the supply conductor IS. The relay is indicated tohave an armature 28 and contacts 29 and 28' for various switchingapplications as may be desirable, depending upon what utilitarianpurpose the amplifier may serve. The two power supplies are merely shownin block diagrams since many different types may be used with equalfacility suchas batteries, or rectified and filtered alternating currentsupplies.

In Fig. 2 the amplifier circuit shown is very similar'to the one in Fig.1, the only modification being in the inverse feedback circuit which isparticularly advantageous for increased stability of operation. It willbe .sufilcient to describe only the changes necessary for obtaininginverse feedback voltage, the remaining part of the circuit beingidentical with the one previously described. It is to be noted that thebias voltage for the grid 3 of the first amplifier tube 5 is nowobtained not only from the bias potential source C through theadjustment of the rider to of the potentiometer 6 but also through theadjustment of the rider 30 of the potentiometer 3| which is in seriesbetween cathode 2| and the common supply conductor 22. In this mandirectcurrent potentials are to be amplified, 50

whereas for alternating current signal amplification, various types ofoutput transformers may be used. 7 I

The principle of operation of the system may be understood by referringto Figure 1 and considering the load resistance IQ of amplifying tube 5in connection with the coupling network comprising resistors IB, l1 andI8. Resistors l1 and I! may be taken as a single resistance element,namely l1 plus l8 and designated as Ra, whereas li as R2 and the loadresistance M as R1. By the use of this simplified designation the"purely numerical identification used in the drawings will not beconfusing in mathematical forms of quantitative expressions. Theconductive coupling R2 and R3 performs a dual function of which thefirst one is the original signal transfer from the plate circuit to thegrid of the succeeding amplifier tube and the second function is toprovide a certain voltage ratio in direct current-potentials whereby thegrid of the tube to which it is connected should be suitably biased at apotential value more negative than its cathode. In other words, theresistors R2 and R3, determine the proper zero signal potential for I2,313,122 connected. The cathode 2| of the amplifier tube the grid 20,which gives the static condition of the amplifier. Considering this, thecathodes of the tubes may be taken as a reference point for I thevarious static potentials, since all cathodes are at substantially thesame potential. Hence. with respect to the cathode I the potential ofthe anode l2 of tube 5 will be of some positive value which may becalled Em and will be approximately one-half of the terminal voltage of10 the supply B, namely Era/2. The potential difference between theanode l2 and the most negative portion of the system at the junctionpointof the resistor I8 and the conductor 9, designated as A, will bethe efiective anode potential plus the potential of the bias source C,that is EP1+Ec. If we denote the potential value whereby the grid ismore negative than the oathode 2l as Eg, then:

R: 29 -m( Pi-lr) C R: is so adjusted that the proper grid potential isobtained to locate the operating point of tube I! near the center of thelinear portion of its transfer characteristic and allowed to remain atthis adjustment. Now, R2 and R3 form a potentiometer between the anodel2 and the point A whose potential is fixed by the supply C. Within theoperating range in which the grid 30 20 draws negligible current (thenormal working range), it is apparent that a fraction of any change inthe potential of the anode I! of tube 5 will be impressed on the grid20, since the voltage relation is determined by Rs r z-t' a which, aswill be seen, represents the efficiency of coupling.

In Equation 1 E may be neglected in comparison with the terms on theright side thereof which then breaks down to the approximate expression:

I I R. E. Efliciency of coupling- R2 E8 wherein R3 represents theresistors l8+l1; R2

resistor I6; Ec the bias voltage supply and Eb the anode voltage supplyof Figure 1.

Now, if

'which can easily be realized in a practical arrangement, then The valueof R2 and Rszshould be high with respect to R1 and thereby the load onthe tube 5 due to the shunting eflfect of the resistors can be madenegligible. As a matter of fact, more so than in the conventionalcapacity coupled amplifiers.

Referring to shown comprises three cascaded stages utilizing preferablyhigh mu triodes in the first two stages and a suitable output tube whichmay be a 7 pentode. In all figures, the heater circuit for the tubes hasbeen omitted for the sake of simplicity, for depending upon the type ofoperation desired, the filaments may be energized by either alternatingor direct current. The power supply shown is the commonly used full waverectifier cal Efliciency Figs. 3 and 4, the amplifier circuit typecircuit for the anode potential side of the system and a half-waverectifier furnishes the negative side of the supply. It is to beunderstood that other types of supplies may be used, or batteries, aslong as the filtering is satisfactory for obtaining pure direct currentand the regulation of the supply adequate for high gain circuits, asshown in Figs. 1 or 2. i

The three-stage amplifier shown in Figs. 3 and 4 illustrates theadaptability of the basic circuit previously described to the cascadingof several stages and to energization from power line operated supplies.Essentially the circuit is similar as to basic details to the one shownin Fig. 1 and the modification illustrated in Fig. 4 includes theinverse feed-back connection in the manner similar as shown in Fig. 2.In both figures the additional voltage amplifying stage has identicalcomponent elements which are marked with the same reference charactersas used in the preceding Stage, except primary indices are afiixed forthe purpose ofddentification of .the additional stage. Other stages maybe inserted in the same manner within practical limitations governed bythe gain obtained and the type of vacuum tubes chosen. The explanationpreviously given regarding the operation of the system applies also tothe circuits shown in these figures.

Additional elements in Figs. 3 and 4 include the power supply for theanode and grid potentials comprising the supply transformer 34, theprimary winding 35 which is to be energized from a suitablealternating-current power line. The secondary winding 31 connects to theanodes 38 and 38 of the rectifier tube 39. The center tap of thesecondary winding is connected to the conductor 22 which forms theneutral point between the two supply voltages.

The extreme positive side of the supply includes a suitable filter chokeB and the conventional filter condensers 4i and 42 between the return ofthe winding 31 and the cathode d3 of tube 39 to which one terminal ofthe filter choke 40 is connected. The extreme negative side' of thesupply is obtained by the half-wave rectifier tube 44, the cathode 45 ofwhich is connected to one terminal of the secondary winding'31, and theanode 61 thereof in series with filter resistor 48 to the supplyconductor 9. Filter condensers 50 and 5| connect to the respectiveterminals of the filter resistor 48 and the supply line 22. The negativeportion of the supply between conductors 22 and 9is also paralleled byvoltage regulator tubes 52 and 53 connected in series.

The output tube is utilized in this arrangement is of the pentode typehaving its suppressor grid 24 connected to the cathode 2|, whereas thescreen grid 21 is connected to the positive side through a suitableresistor 33 which is also connected-to the neutral conductor 22 througha voltage regulator tube 35-, in order that the potential of the screengrid 21 should be maintained uniform. Additional voltage regulationbetween the amplifying stages may be obtained by voltage regulator tubes46 and 49 and series resistors 54 and 58 between the supply conductor l5and the loadresistors l4 and I4. The type of voltage regulator tubes.particularly suitable in this connection may be the neon or similarinert gas The circuit bf Fig. 4, as stated before, is iden-- tical withthe one in Fig. 3, except for the inverse ieed-back circuit whichcomprises a common connection between cathodes I and 2! of tubes 5 andi9 respectively, and variable resistor 31 in series between the cathode2| and the supply conductor 22. The voltage drop across resistor 3|depends upon the plate current variations of tube is and the amount ofthe drop may be regulated by the rider i3 of the resistor 31. Thevoltage obtained will tend to increase the potential of the cathode I toa value more positive with respect to conductor 22, thereby increasingthe negative bias on the grid 3 of vacuum tube 5, tending to reduceamplification. The

fact that all cathodes are at equipotential makes the application. ofinverse feedback very simple in this amplifier. In. this figure anoutput transformer 32 takes the place of the relay winding 26, merely byway of example, illustrating the adaptability of the system to A. C.voltage amplification. a

In a practical embodiment of the circuit shown in Fig. 3, the followingvalues of component elements were found to give good performance.

an anode, a cathode and a control electrode, a voltage source having oneof its terminals connected to said anode through a first impedance, aconnection between the cathode of said first.

tube and an intermediate voltage-point onsaid voltage source, animpedance shunted between the anode of said first tube and the oppositeterminal of said voltage source, a second tube comprising an anode, acathode and a control electrode, a connection between the last-mentionedcontrol electrode and an intermediate point on the last-mentionedimpedance, a load connected between the anode of said second tube andthe first-mentioned terminal of said voltage source, an impedanceconnected between the cathode of said second tube and thefirst-mentioned intermediate voltage-point, an impedance connectedbetween an intermediate point of thelast-men- 05 tioned impedance andsaid other terminal of said voltage source, and a connection between anintermediate point of the last-mentioned impendance and the controlelectrode of said first tube.

WILSON M. B RUBAKER.

